Visual, auditory and somatosensory average evoked responses (AER) were studied in families of patients with affective illness and in normal twins. Relatively high heritabilities were found for some of the measures which also differentiate patient groups from normals. Multivariate techniques have been applied to better define the AER parameters in our twin data sample. In patients with affective illness, affective disorder probands and their siblings showed similar AER amplitude/intensity slopes, as did probands and their spouses. Within the probands' families, the "well" and "ill" relatives were not significantly different, with both groups showing significant augmenting in comparison with normal controls. This suggests that AER stimulus intensity slope measures and affective disorders are not transmitted by a single genetic factor but that AER may be related to increased genetic vulnerability.